The RX1 has only a fixed lens, but it has enough pixels to sufficiently crop great photos from its 24MB shots. So you can do your zooming at the computer. If you are looking at full size DSLRS, I might even consider the Nikon D800 with 36MP and a 50mm or 80mm prime lens. The D800 is much less expensive than the Canon 5D with nearly twice as many pixels. Some compare it with medium format cameras. But you need a good computer to crunch its big pictures. You would also be able to crop a 1/3 of the picture and still have over 10 megapixels. For me I think the RX1 is too expensive for me to spend on a fixed lens camera. But it is more portable than a DSLR.

The Nikon D800 with a prime lens is easier to carry around than a 5D III with a 24-70 zoom lens. DSLRs are big and hard to carry with you all the time. For shooting kids, you should consider the RX100 which is pocketable and shoots stunning 20MP pictures for a point and shoot. And you can have it with you all the time.

If you are intent on Canon, I would seriously consider the 6D. It has geotagging and WIFI which the 5D III does not have.